1. Field of the Invention
This invention relates to a process for vacuum carburizing of steel.
2. Description of the Prior Art
Carburizing is the introduction of additional carbon to the surface of a steel part in order to effect case hardening. In gaseous carburizing, an endothermic gas, which contains carbon monoxide and hydrogen, is used as a carrier gas to displace the air in the furnace.
A hydrocarbon-containing gas such as natural gas, propane or butane is added to the endothermic gas in varying quantities. The carbon monoxide from the endothermic gas and the hydrocarbon react to form nascent carbon atoms, which in turn combine with the iron in the steel to form iron carbide. The iron carbide provides the case.
The steel is exposed to the carburizing atmosphere at high temperatures, e.g., temperatures in the austenitic range for the steel in question, for a predetermined time to achieve the desired depth of carbon penetration into the steel surface. This depth is called the depth of the case.
Carburized or case hardened steel has many important uses because of its desirable properties. The case provides extreme hardness at the surface while the inner portion, or core, beyond the case is relatively soft and ductile. Therefore, case hardened steel has excellent wear properties in combination with the toughness of the core.
In conventional gas carburizing furnaces, a carburizing atmosphere is force circulated by a fan system over the steel in the chamber at atmospheric pressures. Usually, a hydrocarbon gas, such as is found in natural gas, is utilized in combination with a carrier gas, such as an endothermic gas, as the carburizing atmosphere. The carburizing atmosphere is circulated in the furnace for a predetermined time and under predetermined conditions to carburize the steel. The various ramifications and modifications of this technique are well known to those skilled in the art.
The endothermic gas is generally made by cracking natural gas in air to form CO and H.sub.2. Natural gas is expensive, however, and its availability in some areas is uncertain.
Moreover, residual hydrocarbons in natural gas, such as propylene and butylene, are uncrackable. Thus as much as 0.1 to 1% uncracked hydrocarbon may be present in the endothermic gas during the displacement of air, causing soot to form on the workpiece.
The use of lower aliphtic alcohols, i.e., those having 1-4 carbon atoms, principally methanol, as a carrier gas in atmospheric carburizing furnaces avoids the disadvantages of endothermic gas. Methanol is readily available and, under the conditions which exist in carburizing furnaces, breaks down cleanly, forming pure H.sub.2 and CO.
For example, Wyss, U.S. Pat. No. 3,201,290 teaches the use of such lower aliphatic alcohols as methanol and isopropanol as carrier gases. Solomon, U.S. Pat. No. 4,145,232 teaches a process of carburizing steel in a defined atmosphere comprising a carrier gas and a gaseous hydrocarbon, wherein the carrier may be endothermic gas or a nitrogen-methanol or nitrogen-ethanol mixture. It is also disclosed that, although nitrogen and methanol can be introduced separately into the carburizing chamber, they are usually introduced simultaneously.
The process disclosed in the Solomon patent requires a specific carburizing atmosphere having defined amounts of carbon monoxide, hydrogen, nitrogen, carbon dioxide, water vapor and hydrocarbon. The carburizing process is carried out in conventional carburizing chambers at atmospheric pressures with conventional seals.
The process described by the Solomon patent is disadvantageous in that it is an atmospheric process. Thus, air cannot be completely excluded from the furnace. Under carburizing conditions, air can combine with the other gases present and cause a dangerous explosive situation.
Moreover, atmospheric carburizing furnaces have stagnant areas. These stagnant areas do not allow fresh supplies of carburizing gas to flow around the workpiece, causing uneven cases with limited densities. Even when the gas is streamed over the workpiece, it is still not feasible to do so uniformly, especially when the workpiece is closely packed within the furnace.
Vacuum carburizing of steel is likewise known in the art and avoids the problems of atmospheric carburizing. For example, Westeren et al., U.S. Pat. Nos. 3,796,615 and Re. 29,881, Liurque et al., U.S. Pat. No. 4,168,186 and Novy et al., U.S. Pat. No. 4,160,680 teach methodology and apparatus for the carburizing and carbonitriding of steel under vacuum. The process, in essence, comprises evacuating the carburizing chamber, thus drawing air and oxygen away from the steel in the chamber. The steel is heated and the carburizing atmosphere is introduced into the furnace by a partial backfill of natural gas or propane. Cooling and/or quenching after carburizing may be provided by a recirculating cooling gas or by quenching means external to the carburizing chamber.
There is no teaching in any of the references which discloses an alcohol carrier gas to conduct carburization under vacuum instead of atmospherically. There is, likewise, no teaching in any of the references which discloses vacuum carburization to use an alcohol as the carrier gas. The need continues to exist, therefore, for a method of carburizing steel which avoids the problems due to the use of both endothermic gas as the carrier gas and atmospheric furnaces.